This invention relates to a method of making functional devices, such as optical Multiplexers (Mux) and Demultiplexers (Dmux), that have at least one film deposited on a wafer and are subject to a high temperature anneal that induces warping in the wafer.
Multiplexers (Mux) and Demultiplexers (Dmux) need silica waveguides that require the introduction of high performance (i.e. extremely transparent in the 1.50-1.55 wavelength range) silica films on a silicon wafer.
Typically the optical silica films used to form the waveguides are deposited over a silicon wafer by PECVD at a relatively low temperature of 400xc2x0 C. The deposited PECVD silica films do not have sufficiently high performance as deposited due to some absorption peaks causing optical absorption in the 1.50-1.55 wavelength range.
In order to eliminate these residual absorption peaks, an anneal at a high temperature ranging between 700 and 1200xc2x0 C. is required. Because of the difference in thermal expansion between the PECVD deposited silica films and the underlying silicon wafer, the mechanical stress in the silica films increases as the temperature of the silicon-silica films structure is increased and a polarization dependence is observed.
At a temperature exceeding about 600xc2x0 C., the mechanical stress in the silica films reaches a plateau and does not increase as much with any further increase of temperature. As the temperature is increased over 600xc2x0 C., a plastic deformation of the silica films occurs and a stress-temperature hysteresis is observed as the temperature of the silica films/wafer structure is reduced back to room temperature. The consequence of the stress-temperature hysteresis is that the room temperature mechanical stress of the PECVD silica films following the high temperature anneal over 600xc2x0 C. is much more compressive than the room temperature mechanical stress prior to the anneal.
The effect of the induced compressive mechanical stress in the high temperature annealed PECVD silica films is a significant warp of underlying the silicon wafer, which results in a low yield photolithography processing: i.e. the silicon wafer cannot be processed reliably. Moreover, in the event that such a wafer could be processed, the resulting patterned optical silica films show a polarization dependence problem caused by the birefringence originating from this large compressive stress in the silica films.
It is indeed well known that the manufacture of satisfactory Mux/Dmux devices is a very difficult task. Various approaches to the problem have been considered.
The optical silica films can be deposited on the silicon wafer at a very high temperature using the Flame Hydrolysis Deposition (FHD) technique which involves the fusion in hydrogen, oxygen and other gases of fine glass particles followed by some post-deposition anneals to 1200-1350xc2x0 C. (Suzuki S., Polarization insensitive arrayed-waveguide gratings using dopant-rich silica-based glass with thermal coefficient adjusted to silicon substrate, Electron. Lett. 33 (13), 1173, 1997). In this case, the silica films are doped with a high concentration of germanium to increase its Thermal Coefficient of Expansion (TCE) and to try to match the TCE of the underlying silicon wafer, thus minimizing the warp of the underlying silicon wafer (that results from the difference of TCE between the silica films and the silicon wafer) and the associated polarization dependence.
In another similar reference, also using the Flame Hydrolysis Deposition (FHD) technique, a sputter deposited amorphous silicon layer over the silica films (to match the TCE of the underlying silicon wafer) is used to minimize the associated wafer warp and associated polarization dependence of the waveguide. (Takahashi H., Polarization-insensitive arrayed waveguide wavelength multiplexer with birefringence compensating film, IEEE Photon. Tech. Lett. 5 (6), 707, 1993).
In a third similar reference, also using the Flame Hydrolysis Deposition (FHD) technique, a quartz wafer (with a better matching the TCE of the silica films) replaces the silicon wafer as to minimize the residual compressive stress of the silica films after anneals and their polarization dependence. (Kawachi M., Silica waveguides on silicon and their application to integrated-optic components, Optical and quantum Electronics, 22, 391, 1990).
The optical silica films can be grown from silicon at very high temperature using the High Pressure Steam (HPS) technique followed by chemical vapor deposition of phosphorus-doped silica deposition and by a very high temperature anneal at about 1000xc2x0 C. (Verbeek B., Integrated four-channel Mach-Zehnder multi-demultiplexer fabricated with phosphorus doped SiO2 waveguides on Si, J. Lightwave tech., 6 (6), 1011, 1988; Henry C., Four-channel wavelength division multiplexers and bandpass filters on elliptical Bragg reflectors, J. Lightwave tech., 8 (5), 748, 1990; Adar R., Less than 1 dB per meter propagation loss of silica waveguides measured using a ring oscillator, J. Lightwave tech., 12 (8), 1369, 1994) These references do not address the wafer warp problem
The optical silica films can be deposited at a lower temperature of about 350xc2x0 C. by Electron-Beam Vapor Deposition (EBVD) followed by very high temperature anneals at 1200xc2x0 C. (Imoto K., Silica Glass waveguide structure and its implication to a multi/demultiplexer, ECOC, 577, 1988; Imoto K., High-silica guided-wave optical devices, 41st ECTC, 483, 1991).
In these cases the minimization of wafer warp and of the associated polarization dependence is achieved by doping the silica films with a high concentration of germanium or titanium as to match the TCE of the silica films with the underlying quartz wafer.
Other references, using PECVD techniques, describe the need for high temperature anneals of the PECVD silica films as to eliminate the residual optical absorption peaks (Valette S., New integrated optical multiplexer-demultiplexer realized on silicon substrate, ECIO ""87, 145, 1987; Henry C., Glass waveguides on silicon for hybrid optical packaging, J. Lightwave tech., 7 (10), 1350, 1989; Grand G., Low-loss PECVD silica channel waveguides for optical communications, Electron. Lett., 26 (25), 2135, 1990; Bruno F., Plasma-enhanced chemical vapor deposition of low-loss SiON optical waveguides at 1.5-xcexcm wavelength, Applied Optics, 30 (31), 4560, 1991; Lai Q., Simple technologies for fabrication of low-loss silica waveguides, Elec. Lett., 28 (11), 1000, 1992; Lai Q., Formation of optical slab waveguides using thermal oxidation of SiOx, Elec. Lett., 29 (8), 714, 1993; Liu K., Hybrid optoelectronic digitally tunable receiver, SPIE, Vol 2402, 104, 1995; Tu Y., Single-mode SiON/SiO2/Si optical waveguides prepared by plasma-enhanced Chemical vapor deposition, Fiber and integrated optics, 14, 133, 1995; Hoffmann M., Low temperature, nitrogen doped waveguides on silicon with small core dimensions fabricated by PECVD/RIE, ECIO""95, 299, 1995; Poenar D., Optical properties of thin film silicon-compatible materials, Appl. Opt. 36 (21), 5112, 1997; Hoffmann M., Low-loss fiber-matched low-temperature PECVD waveguides with small-core dimensions for optical communication systems, IEEE Photonics Tech. Lett., 9 (9), 1238, 1997; Pereyra I., High quality low temperature DPECVD silicon dioxide, J. Non-Crystalline Solids, 212, 225, 1997; Kenyon T., A luminescence study of silicon-rich silica and rare-earth doped silicon-rich silica, Fourth Int. Symp. Quantum Confinement Electrochemical Society, 97-11, 304, 1997; Alayo M., Thick SiOxNy and SiO2 films obtained by PECVD technique at low temperatures, Thin Solid Films, 332, 40, 1998; Bulla D., Deposition of thick TEOS PECVD silicon oxide layers for integrated optical waveguide applications, Thin Solid Films, 334, 60, 1998; Valette S., State of the art of integrated optics technology at LETI for achieving passive optical components, J. of Modern Optics, 35 (6), 993, 1988; Ojha S., Simple method of fabricating polarization-insensitive and very low crosstalk AWG grating devices, Electron. Lett., 34 (1), 78, 1998; Johnson C., Thermal annealing of waveguides formed by ion implantation of silica-on-Si, Nuclear Instruments and Methods in Physics Research, B141, 670, 1998; Ridder R., Silicon oxynitride planar waveguiding structures for application in optical communication, IEEE J. of Sel. Top. In Quantum Electron., 4 (6), 930, 1998; Germann R., Silicon-oxynitride layers for optical waveguide applications, 195th meeting of the Electrochemical Society, 99-1, May 1999, Abstract 137, 1999; Worhoff K., Plasma enhanced cyhemical vapor deposition silicon oxynitride optimized for application in integrated optics, Sensors and Actuators, 74, 9, 1999; Offrein B., Wavelength tunable optical add-after-drop filter with flat passband for WDM networks, IEEE Pphotonics Tech. Lett., 11 (2), 239, 1999).
Amazingly, only one of these references (Ojha, 1998) addresses the upper mentioned wafer warp and polarization dependence problems by mentioning that the elimination of these was achieved by adjusting the TCE of the silica glass to the silicon wafer by increasing its doping with large concentrations of germanium, boron or phosphorus.
According to the present invention there is provide a method of making a device having at least one deposited layer and subject to a post deposition high temperature anneal, comprising the steps of providing a wafer having a front and back face; depositing opposing films on each said face of said wafer, said opposing films having similar mechanical properties; and subsequently subjecting said wafer to said high temperature anneal, whereby said opposing films tend to cancel out stress-induced warping of said wafer during said anneal.
The high temperature anneal should typically be carried out at a temperature above about 600xc2x0 C., preferably between 600 and 900xc2x0 C.
The opposing films should have similar mechanical properties so that the stress induced effects are canceled out as the wafer is subjected to the high temperature anneal. Preferably this is achieved by making them the same composition and thickness, but it would be theoretically possible to use different films so long as the mechanical effects canceled out.
In a preferred aspect, the invention involves the use of a special technique involving the deposition of thick PECVD silica films on the back face of the silicon wafer in order to prevent the wafer warp problem following these required high temperature anneals and to achieve a stable manufacturing of high performance high temperature annealed PECVD optical silica films with lower polarization dependence.
The invention also teaches a technique for introducing Plasma Enhanced Chemical Vapor Deposition (PECVD) silica waveguides in Mux and Dmux optical devices while eliminating the wafer warp problem and minimising the polarization dependence problem (caused by birefringence, i.e. the pass wavelengths of the Mux differ for the TE and TM modes) associated with the high temperature anneals that are required in order to reduce the optical absorption of thick optical silica films serving as waveguides.
The optical silica films deposited on a silicon wafer by PECVD at a relatively low temperature of 400xc2x0 C. may also require a further post-deposition anneal at a high temperature ranging between 700 and 1200xc2x0 C. in order to eliminate the residual optical absorption peaks.
A unique technical aspect of this invention is the deposition of thick PECVD silica films on the back face of the silicon wafer in order to prevent the wafer warp problem and to minimize the polarization dependence following these required high temperature anneals so as to achieve the stable manufacture of high performance high temperature annealed PECVD optical silica films with lower polarization dependence. The silica film on the back face compensates for the stress induced warp arising during the high temperature anneal caused by the silica film on the front face.
In a preferred embodiment, a temporary protective film is deposited on the front face. This is etched away after deposition of the film on the back face. During this etch step, the film on the back face also gets etched, so the initial thickness of the film on the back face is increased by an amount equal to the portion of the deposited layer that is etched away during this subsequent etch step so that the final thickness of the film on the back face is the same as the thickness of the film deposited on the front face.
The invention also provides a functional device, comprising a wafer having front and back faces; a layer deposited on the front face of said wafer to provide said device with its functional properties; and a matching layer deposited on the back face of said wafer to compensate for stress induced warp during an anneal step in the manufacture of said device.